A new human oncogene derived from the DNA of the spleen of a patient with a poorly differentiated B lymphocytic lymphoma has recently been cloned in cosmids in our laboratory. We have begun characterization of two cosmid clones by restriction endonuclease mapping of the oncogene. The first clone, designated C-8-X-1-1, has been shown to contain both human and mouse (NIH/3T3)-derived sequences. The human sequences of the C-8-X-1-1 (which had previously been shown to be devoid of biological activity, i.e., transformation of NIH/3T3 cells), have been shown to overlap partially with the human sequences contained in the cosmid clone C-14-1-2, which was shown to possess transforming activity. This last clone has been partially characterized as far as the enzyme restriction map is concerned and several fragments of it have been purified and subcloned either in pBR322 or in pAT-153. Some of the fragments, which are almost completely free of repetitive human sequences (Alu-negative), have been used to screen a normal human genomic library in order to detect sequences homologous to the new oncogene in normal human DNA. Three phages containing sequences homologous to some of the isolated fragments have been isolated so far and their further characterization is currently being performed. The Alu-negative fragments isolated from the cloned oncogene, as well as some Alu-containing sequences, also isolated by us, are being used to detect the mRNA for the gene in transfected NIH/3T3 cells and will be used to screen fresh normal and tumor tissues for expression of the newly isolated oncogene.